Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Assets [Abstract]
Cash and cash equivalents	$ 920.3	$ 1,150.3	$ 1,007.8	$ 1,303.2
Restricted cash	53.7		16.5
Inventories:
Construction in progress and finished homes	4,907.8		4,606.0
Residential land and lots - developed and under development	4,827.7		4,519.7
Land held for development	100.5		101.0
Land held for sale	204.2		10.4
Total inventories	10,040.2		9,237.1
Investment in unconsolidated entities	86.1		0.0
Mortgage loans held for sale	538.2		587.3
Deferred income taxes	239.1		365.0
Property and equipment, net	357.7		325.0
Other Assets	622.0		565.9
Goodwill	100.0		80.0
Total assets	12,957.3		12,184.6
Liabilities [Abstract]
Accounts payable	575.7		580.4
Accrued expenses and other liabilities	1,068.1		985.0
Notes payable	3,258.1		2,871.6
Total liabilities	4,901.9		4,437.0
Revenues
Home sales	3,184.5	2,797.7
Land/lot sales and other	67.2	28.4
Financial services	81.0	78.1
Total revenues	3,332.7	2,904.2
Cost of sales
Home sales	2,521.5	2,244.8
Land/lot sales	54.9	20.8
Inventory and land option charges	3.7	2.3
Total cost of sales	2,580.1	2,267.9
Selling, General and Administrative Expense	384.2	325.9
Equity in earnings of unconsolidated entities	(2.3)	0.0
Interest Expense	0.0
Other (income) expense	(20.5)	(7.7)
Income before income taxes	391.2	318.1
Depreciation and amortization	16.2	14.4
Net cash (used in) provided by operating activities	(75.0)	(28.2)
Segment Reconciling Items [Member]
Assets [Abstract]
Cash and cash equivalents	0.0
Restricted cash	0.0
Inventories:
Construction in progress and finished homes	0.0
Residential land and lots - developed and under development	46.7
Land held for development	0.0
Land held for sale	2.8
Total inventories	49.5
Investment in unconsolidated entities	21.0
Mortgage loans held for sale	0.0
Deferred income taxes	0.3
Property and equipment, net	0.0
Other Assets	20.4
Goodwill	20.0
Total assets	111.2
Liabilities [Abstract]
Accounts payable	0.0
Accrued expenses and other liabilities	(23.8)
Notes payable	12.6
Total liabilities	(11.2)
Revenues
Home sales	0.0
Land/lot sales and other	0.0
Financial services	0.0
Total revenues	0.0
Cost of sales
Home sales	0.0
Land/lot sales	4.4
Inventory and land option charges	0.0
Total cost of sales	4.4
Selling, General and Administrative Expense	0.1
Equity in earnings of unconsolidated entities	5.3
Interest Expense	(2.1)
Other (income) expense	0.0
Income before income taxes	(7.7)
Depreciation and amortization	0.1
Net cash (used in) provided by operating activities	(8.1)
Homebuilding [Member]
Assets [Abstract]
Cash and cash equivalents	558.0		973.0
Restricted cash	8.3		9.3
Inventories:
Construction in progress and finished homes	4,907.8		4,606.0
Residential land and lots - developed and under development	4,649.2		4,519.7
Land held for development	100.5		101.0
Land held for sale	18.2		10.4
Total inventories	9,675.7		9,237.1
Investment in unconsolidated entities	0.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes	236.3		365.0
Property and equipment, net	204.3		194.4
Other Assets	545.3		518.7
Goodwill	80.0		80.0
Total assets	11,307.9		11,377.5
Liabilities [Abstract]
Accounts payable	567.0		575.6
Accrued expenses and other liabilities	997.2		933.1
Notes payable	2,749.6		2,451.6
Total liabilities	4,313.8		3,960.3
Revenues
Home sales	3,184.5	2,797.7
Land/lot sales and other	36.4	28.4
Financial services	0.0	0.0
Total revenues	3,220.9	2,826.1
Cost of sales
Home sales	2,521.5	2,244.8
Land/lot sales	31.2	20.8
Inventory and land option charges	3.7	2.3
Total cost of sales	2,556.4	2,267.9
Selling, General and Administrative Expense	304.8	268.4
Equity in earnings of unconsolidated entities	0.0
Interest Expense	0.0
Other (income) expense	(14.1)	(4.1)
Income before income taxes	373.8	293.9
Depreciation and amortization	13.1	13.3
Net cash (used in) provided by operating activities	(101.6)	(98.3)
Forestar Group [Member]
Assets [Abstract]
Cash and cash equivalents	321.8
Restricted cash	40.0
Inventories:
Construction in progress and finished homes	0.0
Residential land and lots - developed and under development	131.8
Land held for development	0.0
Land held for sale	183.2
Total inventories	315.0
Investment in unconsolidated entities	65.1
Mortgage loans held for sale	0.0
Deferred income taxes	2.5
Property and equipment, net	2.0
Other Assets	18.4
Goodwill	0.0
Total assets	764.8
Liabilities [Abstract]
Accounts payable	2.4
Accrued expenses and other liabilities	45.5
Notes payable	108.4
Total liabilities	156.3
Revenues
Home sales	0.0
Land/lot sales and other	30.8
Financial services	0.0
Total revenues	30.8
Cost of sales
Home sales	0.0
Land/lot sales	19.3
Inventory and land option charges	0.0
Total cost of sales	19.3
Selling, General and Administrative Expense	13.6
Equity in earnings of unconsolidated entities	(7.6)
Interest Expense	2.1
Other (income) expense	(0.6)
Income before income taxes	4.0
Depreciation and amortization	1.2
Net cash (used in) provided by operating activities	(36.2)
Financial Services [Member]
Assets [Abstract]
Cash and cash equivalents	24.5		24.1
Restricted cash	5.4		7.2
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots - developed and under development	0.0		0.0
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	0.0		0.0
Investment in unconsolidated entities	0.0
Mortgage loans held for sale	538.2		587.3
Deferred income taxes	0.0		0.0
Property and equipment, net	3.1		3.0
Other Assets	34.1		42.2
Goodwill	0.0		0.0
Total assets	605.3		663.8
Liabilities [Abstract]
Accounts payable	1.6		1.5
Accrued expenses and other liabilities	30.9		35.6
Notes payable	387.5		420.0
Total liabilities	420.0		457.1
Revenues
Home sales	0.0	0.0
Land/lot sales and other	0.0	0.0
Financial services	81.0	78.1
Total revenues	81.0	78.1
Cost of sales
Home sales	0.0	0.0
Land/lot sales	0.0	0.0
Inventory and land option charges	0.0	0.0
Total cost of sales	0.0	0.0
Selling, General and Administrative Expense	61.7	54.8
Equity in earnings of unconsolidated entities	0.0
Interest Expense	0.0
Other (income) expense	(2.9)	(3.2)
Income before income taxes	22.2	26.5
Depreciation and amortization	0.4	0.3
Net cash (used in) provided by operating activities	67.9	59.9
Other Segments [Member]
Assets [Abstract]
Cash and cash equivalents	16.0		10.7
Restricted cash	0.0		0.0
Inventories:
Construction in progress and finished homes	0.0		0.0
Residential land and lots - developed and under development	0.0		0.0
Land held for development	0.0		0.0
Land held for sale	0.0		0.0
Total inventories	0.0		0.0
Investment in unconsolidated entities	0.0
Mortgage loans held for sale	0.0		0.0
Deferred income taxes	0.0		0.0
Property and equipment, net	148.3		127.6
Other Assets	3.8		5.0
Goodwill	0.0		0.0
Total assets	168.1		143.3
Liabilities [Abstract]
Accounts payable	4.7		3.3
Accrued expenses and other liabilities	18.3		16.3
Notes payable	0.0		0.0
Total liabilities	23.0		19.6
Revenues
Home sales	0.0	0.0
Land/lot sales and other	0.0	0.0
Financial services	0.0	0.0
Total revenues	0.0	0.0
Cost of sales
Home sales	0.0	0.0
Land/lot sales	0.0	0.0
Inventory and land option charges	0.0	0.0
Total cost of sales	0.0	0.0
Selling, General and Administrative Expense	4.0	2.7
Equity in earnings of unconsolidated entities	0.0
Interest Expense	0.0
Other (income) expense	(2.9)	(0.4)
Income before income taxes	(1.1)	(2.3)
Depreciation and amortization	1.4	0.8
Net cash (used in) provided by operating activities	3.0	10.2
East [Member] | Homebuilding [Member]
Inventories:
Total inventories	1,155.6		1,068.9
Revenues
Total revenues	393.0	305.9
Cost of sales
Income before income taxes	45.0	26.3
Midwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	543.3		492.6
Revenues
Total revenues	161.4	151.1
Cost of sales
Income before income taxes	13.3	10.2
Southeast [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,470.2		2,392.3
Revenues
Total revenues	988.7	883.4
Cost of sales
Income before income taxes	122.5	99.6
South Central [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,281.9		2,199.4
Revenues
Total revenues	808.8	756.9
Cost of sales
Income before income taxes	101.5	96.5
Southwest [Member] | Homebuilding [Member]
Inventories:
Total inventories	516.7		506.1
Revenues
Total revenues	156.4	108.6
Cost of sales
Income before income taxes	14.7	4.0
West [Member] | Homebuilding [Member]
Inventories:
Total inventories	2,477.7		2,352.5
Revenues
Total revenues	712.6	620.2
Cost of sales
Income before income taxes	76.8	$ 57.3
Corporate, Non-Segment [Member] | Homebuilding [Member]
Inventories:
Total inventories	$ 230.3		$ 225.3